Signal transmission mechanism with diversity gain in satellite communication network

ABSTRACT

Provided is a satellite communication system for obtaining a diversity gain by transmitting a signal applied with STBC. When a signal transmission band of a link between a satellite and a terminal differs from that of a link between a satellite and a repeater, the satellite communication system includes: a satellite for transmitting a first signal encoded to STBC using a band of a link formed between the satellite to a terminal and transmitting a second signal encoded to STBC using a band of a link formed between the satellite to a repeater; and at least one of frequency transforming repeaters for receiving the second signal through the band of the link formed between the satellite to the repeater, transforming the received second signal to a signal with other frequency band, and transmitting the transformed second signal using a band of a link formed to a terminal.

TECHNICAL FIELD

The present invention relates to a method of transmitting a signal froma satellite and a repeater in a satellite communication network and amethod of receiving and processing signals from a satellite and arepeater at a terminal; and more particularly, to a satellitecommunication system for enabling a terminal to obtain diversity gain bytransmitting a signal applied with a space time block code (STBC)through regarding a satellite and a repeater as two transmittingantennas, and a method for detecting a signal by performing STBCdecoding on signals received from the satellite and the repeaterregarded as two transmitting antennas and estimating a signaltransmitting/receiving channel at the terminal.

BACKGROUND ART

The development of communication technology makes possible to providevarious signals such as video, audio and data to users through asatellite although the users are moving around. In order to eliminate ashadow area in a satellite communication network, repeaters areinstalled at the ground.

The satellite digital multimedia broadcasting (DMB) in Republic of Koreaand the satellite-digital multimedia broadcasting (S-DMB) in Europe arerepresentative satellite communication system including repeaters. Arepeater is defined as gap-filler in the satellite DMB of Republic ofKorea, and a repeater is defined as an intermediate module repeater inthe S-DMB of Europe. In a satellite communication network, a repeater isclassified as a simple amplifying repeater and as a frequencytransforming repeater. The simple amplifying repeater receives a signalfrom a satellite, amplifies the received signal in the same band, andrelays the amplified signal to a terminal. The frequency transformingrepeater transforms the frequency of the signals received from thesatellite, and relays the transformed signals to a terminal.

In the terminal of the satellite communication network, multipath fadingoccurs to receive more than two signals. In order to stably receive themulti path fading signals, the terminal includes a CDMA rake receiver.

In the satellite communication network, a satellite simply radiatessignals to the ground. A repeater simply amplifies a signal and relaysthe amplified signal, or transforms a signal to a signal with otherfrequency band and relays the transformed signal. Therefore, a terminalcannot obtain diversity gain through signals received from the satelliteand from the repeaters.

For example, if a space time block code (STBC) defined in 3GPP WCDMAbased IMT-2000 wireless interface specification is used, multipathfading can be reduced without requiring additional bandwidth in a multiantenna system, thereby obtaining diversity gain.

However, the STBC scheme cannot be applied to the conventional signaltransmission scheme used in the satellite and the repeaters in thesatellite communication network. That is, in order to apply the STBCscheme, a transmitting end is required to have two transmitting antennasand transmits time space block codes through the two transmittingantennas, and a terminal must be capable of obtaining time and spacecorrelation of signals received from different antennas. Therefore, itis impossible to apply the STBC scheme into the conventional signaltransmission scheme used in the satellite communication network.

Therefore, there is demand for a method to apply the space time blockcode (STBC) scheme into a satellite communication system without agapfiller. Especially, there is demand for a method for obtainingdiversity gain at a terminal by transmitting signal with STBC appliedthrough reading a satellite and a repeater as two transmitting antennas,and a method for making a terminal to receive signals from a satelliteand a repeater like receiving two signals from one transmitting end.

Furthermore, there is demand for a method for detecting a correspondingsignal at a terminal by decoding a STBC coded signal received from asatellite and a repeater, and a method for improving efficiency ofreceiving a signal by performing channel estimation, channelcompensation and channel prediction at a terminal.

DISCLOSURE OF INVENTION Technical Problem

It is, therefore, an object of the present invention to a satellitecommunication system for enabling a terminal to obtain diversity gain bytransmitting a signal applied with a space time block code (STBC)through regarding a satellite and a repeater as two transmittingantennas, and a method for detecting a signal by performing STBCdecoding on signals received from the satellite and the repeaterregarded as two transmitting antennas and estimating a signaltransmitting/receiving channel at the terminal.

Technical Solution

In accordance with one aspect of the present invention, there isprovided a satellite communication system for obtaining a diversity gainby transmitting a signal applied with a space and time block code (STBC)when a signal transmission band of a link between a satellite and aterminal differs from that of a link between a satellite and a repeater,including: a satellite for transmitting a first signal encoded to STBCusing a band of a link formed between the satellite to a terminal andtransmitting a second signal encoded to STBC using a band of a linkformed between the satellite to a repeater; and at least one offrequency transforming repeaters for receiving the second signal throughthe band of the link formed between the satellite to the repeater,transforming the received second signal to a signal with other frequencyband, and transmitting the transformed second signal using a band of alink formed to a terminal.

In accordance with another aspect of the present invention, there isprovided a satellite communication system for obtaining a diversity gainby transmitting a signal applied with a space and time block code (STBC)when a signal transmission band of a link between a satellite and aterminal is identical to that of a link between a satellite and arepeater, including: a satellite for transmitting a first signal encodedto STBC using a band of a link formed between the satellite to aterminal and using a band of a link formed between the satellite to eachrepeater, respectively; and at least one of simple amplifying repeatersfor receiving the first signal through the band of the link formedbetween the satellite and the repeater, transforming the received firstsignal to a second signal having the same frequency band of the firstsignal, and transmitting the transformed second signal using a band of alink formed to a terminal.

In accordance with still another aspect of the present invention, thereis provided a satellite communication system for obtaining a diversitygain by transmitting a signal applied with a space and time block code(STBC) when a satellite communication network include repeaters having asimple amplifying function, including: a satellite for transmitting afirst signal encoded to STBC using a band of a link formed between thesatellite to a terminal and using a band of a link formed between thesatellite to each repeater, respectively; at least one of first simpleamplifying repeaters for receiving the first signal through the band ofthe link formed to the satellite, amplifying the received first signal,and transmitting the amplified first signal using a band of a linkformed to a terminal; and at least one of second simple amplifyingrepeater for receiving the first signal through the band of the linkformed to the satellite, transforming the first signal to a secondsignal having the same frequency band of the first signal, amplifyingthe transformed second signal, and transmitting the amplified secondsignal using a band of a link formed to a terminal.

In accordance with further still another embodiment of the presentinvention, there is provided a satellite communication system forobtaining a diversity gain by transmitting a signal applied with a spaceand time block code (STBC) when a satellite communication networkinclude repeaters having a frequency transforming function and a signaltransmission band of a link between a satellite and a terminal differsfrom that of a link between a satellite and a repeater, including: asatellite for transmitting a first signal encoded to STBC using a bandof a link between the satellite to a terminal and transmitting a secondsignal that is encoded to STBC using a band of a link formed between thesatellite to a repeater; at least one of first frequency transformingrepeaters for receiving the second signal through the band of the linkto the satellite, transforming the received second signal to a secondsignal having a frequency band corresponding to the link between thesatellite and a terminal, and transmitting the transformed second signalusing a band of a link formed to a terminal; and at least one of secondfrequency transforming repeater for receiving the second signal throughthe band of the link formed to the satellite, transforming the receivedsecond signal to a first signal having a frequency band corresponding toa link between the satellite and the terminal, and transmitting thetransformed first signal using a band of a link formed to a terminal.

In accordance with yet another embodiment of the present invention,there is provided a method for detecting a signal in a terminal in asatellite communication system for obtaining a diversity gain bytransmitting a signal applied with space time block code (STBC) from asatellite and a repeater to the terminal, the method including the stepsof: estimating channel information corresponding to each received signalwhen receiving a signal encoded to STBC from a satellite and a repeaterat the same time; generating a channel matrix using the estimatedchannel information; and

detecting a transmitting signal by performing STBC decoding on thereceived signal based on STBC encoding information previously receivedfrom a satellite and a repeater and the generated channel matrix.

In accordance with yet another embodiment of the present invention,there is provided a method for estimating a channel in a terminal in asatellite communication system for obtaining a diversity gain bytransmitting a signal applied with space time block code (STBC) from asatellite and a repeater to the terminal, the method including the stepsof: estimating channel information value by performing a predeterminedchannel estimation scheme on signals received from a satellite and arepeater; delaying the estimated channel information value as much as D′applying to the delayed channel information value, and subtracting theresult of applying the coefficient α from the estimated channelinformation value; and delaying the subtracted channel information valueas much as D, applying to the delayed channel information value, andadding the result of applying the coefficient β to the subtractedchannel information value.

Advantageous Effects

According to the present invention, STBC can be applied to a satellitecommunication network by only modifying a signal transmission schemeusing in a satellite and a repeater without additionally equipping newapparatus in a satellite communication system. Accordingly, temporal andspatial diversity gain can be obtained.

Also, a terminal is not required to include a new module or chipadditionally to obtain a temporal and spatial diversity gain accordingto the present invention. Therefore, the cost of the terminal can bereduced.

Furthermore, channel estimation, compensation and prediction can beaccurately performed even in a satellite communication networkenvironment according to the present invention although the satellitecommunication network environment has a long round-trip delay time.Moreover, the load of terminal to perform the channel estimation can bereduced according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of the preferredembodiments given in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a diagram illustrating a satellite communication system inaccordance with an embodiment of the present invention;

FIG. 2 is a diagram illustrating transmitting signals transmitted from asatellite and repeaters in accordance with an embodiment of the presentinvention when the transmission band of a link between a satellite and aportable terminal differs from that of a link between a satellite and arepeater;

FIG. 3 is a diagram illustrating signals transmitted from a satelliteand repeaters in accordance with an embodiment of the present inventionwhen signal transmission bands of a link between a satellite and aportable terminal and a link between a satellite and a repeater aresame;

FIG. 4 is a diagram illustrating signals transmitted from a satelliteand repeaters in accordance with an embodiment of the present inventionfor describing that a transmitting method according to the presentembodiment can be applied to a satellite communication networksincluding a simple amplifying repeater and/or a frequency transformrepeater;

FIG. 5 is a flowchart illustrating a method of detecting a signal byperforming STBC decoding on signals received from a satellite andrepeaters at a portable terminal employing a WCDMA based IMT-2000wireless interface specification in accordance with an embodiment of thepresent invention; and

FIG. 6 is a block diagram for describing channel estimation at aterminal in accordance with an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a repeater according to an embodiment of the presentinvention will be described in brief.

A frequency transforming repeater according to the present embodimentbasically includes a frequency transforming function, and denotes arepeater selectively performing a signal processing function, forexample, a signal transforming function, according to the environment ofa satellite communication network. For example, the frequencytransforming repeater according to an embodiment of the presentinvention transforms a signal u₂ received from a satellite to a signalu₂ with other frequency band without processing the received signal,and/or transforms a signal u₁ received from the satellite to a signal u₂with the same frequency band of the signal u₁ by processing the receivedsignal u₁.

Meanwhile, a simple amplifying repeater according to an embodiment ofthe present invention basically includes a signal amplifying function,and denotes a repeater selectively performing a signal processingfunction, for example, a signal transforming function, according to theenvironment of the satellite communication network. For example, thesimple amplifying repeater amplifies a signal u₁ received from asatellite without processing the received signal and/or transforms thesignal u₁ to a signal u₂ with the same frequency band of the signal u₁by processing the received signal u₁.

Other objects and aspects of the invention will become apparent from thefollowing description of the embodiments with reference to theaccompanying drawings, which is set forth hereinafter.

FIG. 1 is a diagram illustrating a satellite communication system inaccordance with an embodiment of the present invention.

As shown in FIG. 1, the satellite communication system includes asatellite 10, repeaters 11 to 13, a base station 80, a contents providerCP 81 and a user terminal such as a portable terminal 60, a stationaryterminal 61, and a mobile terminal 62. Hereinafter, the portableterminal 60 is used as the representative one of the user terminal todescribe the satellite communication system.

In addition, the repeaters in the satellite communication networkincludes a simple amplifying repeater for simply amplifying a signalfrom a satellite and relaying the amplified signal to a terminal, and afrequency transforming repeater for transforming the frequency of asignal from a satellite and relaying the transformed signal to apredetermined terminal. The simple amplifying repeater and frequencytransforming repeater are installed in consideration with acorresponding area of the communication environment. For example, thesimple amplifying repeater is installed for covering a narrow area up to500 m based on LOS, and the frequency transforming repeater is installedfor covering a comparative wider area up to 3 Km.

The present invention provides a method of transmitting a signal at asatellite 10 and repeaters 11 to 13, and a method of receiving a signalat a satellite 10 and repeaters 11 to 13 in order to obtain spatial andtemporal diversity gain form a signal received at a user terminal byapplying a space time block code (STBC) scheme to a satellitecommunication system.

Hereinafter, how to transmit and receive a signal in a satellitecommunication network will be described with reference to FIG. 1 beforedescribing embodiments of the present invention.

The base station 80 transforms contents data provided from the contentsprovider 81 to a signal proper for satellite communication, andtransmits the transformed signal to the satellite 10. The satellite 10transmits the signal directly to the portable terminal 60 or indirectlyto the portable terminal through the repeaters 11 to 13.

As shown in FIG. 1, a link A denotes a link between the base station 80and the satellite 1, and links B1, B2, and B3 denote links between thesatellite 10 and the repeaters 11 to 13. A link C denotes a link betweenthe satellite 10 and the portable terminal 60. Links D1, D2, and D3denote links between the portable terminal and the repeaters 11 to 13.These links may be a bi-directional link or a one-directional linkaccording to embodying specifications of a satellite communicationsystem.

In a satellite digital multimedia broadcasting (DMB) defined in Republicof Korea as an example, the link A is a link with 14.11 GHz band, thelinks B1, B2, and B3 are links with 2.6 GHz band, and the links D1, D2,and D3 are links with 2.6 GHz band. In a S-DMB of Europe as anotherexample, the link A is a link with HDFSS band from 19.6 to 20.2 GHz, thelinks B1, B2, and B3 are links with HDFSS band or satellite IMT-2000band, and the links D1, D2, and D3 are links with a satellite IMT-2000band.

As described above, the object of the present invention is to apply aSTBC scheme to a satellite communication network in order to obtain atemporal and spatial diversity gain from signals received from thesatellite 10 and from the repeaters 11 to 13 at the portable terminal60.

Space time block code (STBC) is defined at 3GPP terrestrial WDCMA basedIMT-2000 wireless interface specification, and is expected to beutilized in future IMT-2000 Advanced specification. Since the currenttechnical specification of satellite communication system hascommonality with the technical specification of terrestrial mobilecommunication system, the portable terminal 60 already employs the STBCscheme. Therefore, it is obvious to those skilled in the art thatadditional modules or chips are not required to be mounted in theportable terminal 60 to apply the STBC scheme into the satellitecommunication network according to the present invention.

The present invention may be applied to a satellite communicationnetwork including no repeaters. If a satellite transmits a signalencoded to STBC through a plurality of antennas without transmitting theSTBC encoded signals through the repeaters, signals from thetransmitting antennas have long round-trip delay times so each signal isnot independently received. Therefore, no benefit is obtained byapplying the STBC scheme in the satellite communication system dislikethe terrestrial mobile communication system. Accordingly, a satellitecommunication network without repeaters is not discussed in the presentinvention.

As described above, the object of the present invention is to apply theSTBC scheme into a satellite communication network because signalsreceived from repeaters 11 to 13 at the mobile terminal 60 haveindependent characteristics.

In order to apply the STBC scheme into the satellite communicationnetwork, a transmitter must regard a satellite and repeaters as twotransmitting antennas, and the satellite and the repeaters transmitsignals encoded to STBC to a portable terminal. Since the portableterminal receives signals from the satellite and the repeaters, itrequires the portable terminal processes the received signals likeprocessing signals received from two transmitting antennas. Hereinafter,a method of transmitting a signal from a satellite and a repeater willbe described with reference to FIGS. 2 to 4. That is, how to modify asignal transmission scheme in a satellite communication network will bedescribed.

In order to apply the STBC scheme into a satellite communicationnetwork, a pilot signal or a channel estimation method is required forestimating a channel from two transmitting antennas to a receivingantenna. A portable terminal supporting the IMT-2000 and IMT-Advancedservice already employs the STBC scheme. For example, the portableterminal uses a frame structure having a pilot signal or a well-knownchannel estimation method in order to estimate channels to twotransmitting antennas. Therefore, it is obvious to those skilled in theart that the portable terminal will not have overhead although the STBCscheme is applied to the satellite communication network.

In order to apply the STBC scheme to a satellite communication network,a portable terminal needs to synchronize a signal received from arepeater with another signal received from a satellite. However, it isobvious to those skilled in the art that these signals can be easilysynchronized by pre-compensating a signal from a satellite at a repeaterin consideration with a process delay time for amplification andfrequency transformation in order to allow a portable terminal toreceive signals from the satellite and the repeater at the same time.

Hereinafter, a method of transmitting a signal at a satellite and arepeater according to the relation of a link between a satellite and aportable terminal, and between a satellite and a repeater for making amobile terminal to receive the signals from the satellite and therepeaters like receiving signals from two transmitting antenna in orderto apply the STBC scheme to a satellite communication network will bedescribed with reference to FIGS. 2 to 4.

FIG. 2 is a diagram illustrating transmitting signals transmitted from asatellite and repeaters in accordance with an embodiment of the presentinvention when the transmission band of a link between a satellite and aportable terminal differs from that of a link between a satellite and arepeater.

It assumes that a transmitter includes two transmitting antennas, afirst antenna 1 and a second antenna 2. Also, the first antenna 1transmits a STBC applied signal u₁ that is encoded to STBC, and thesecond antenna 2 transmits a STBC applied signal u₂.

The method of transmitting signals shown in FIG. 2 is the simplesttransmission method. When a signal transmission band of a link C betweena satellite 20 and a portable terminal 60 differs from that of links B1,B2, and B3 between a satellite and repeaters, that is, when a frequencytransforming repeater is used, the satellite 20 transmits a signal u₁with a band of link C, and the repeaters transmit a signal u₂ with aband of links D1, D2, and D3 by treating the satellite 20 as the firstantenna 1, and the repeaters as the second antenna 2.

Accordingly, the portable terminal 60 receives the signal u₁ from thesatellite 20 and the signal u₂ from the repeaters 21 to 23 at the sametime, and estimates channels of two STBC-encoded signals using a pilotor a known estimation method. Then, the portable terminal 60 detects acorresponding signal by performing STBC decoding.

In the transmission method shown in FIG. 2, the satellite 20 encodes asignal u₁ loaded on the band of the link C, and encodes a signal u₂loaded on the bands of the links B1, B2, and B3 by performing STBCencoding, and transmits the encoded signals u₁ and u₂. The portableterminal 60 can discriminate the signal u₁ and the signal u₂ throughchannel estimation and STBC decoding.

In the transmission method shown in FIG. 2, the repeaters 21 to 23receive a signal u₂ from the satellite 20 through a band of a linkbetween the repeater and the satellite, transform the received signal u₂to a signal with different frequency band, and transmits the transformedsignal u₂ to the portable terminal through a link formed between therepeater and the portable terminal 60. By the above scheme, temporal andspatial diversity gain can be obtained without increment of system costand signal processing time because the load of processing signal doesnot increase in a satellite communication network. However, thetransmission method shown in FIG. 2 cannot provide large diversity gaincompared to a typical method when the link C is not formed as like anurban environment or the intensity of a transmitting signal is veryweak.

FIG. 3 is a diagram illustrating signals transmitted from a satelliteand repeaters in accordance with an embodiment of the present inventionwhen signal transmission bands of a link between a satellite and aportable terminal and a link between a satellite and a repeater aresame, that is, when a simple amplifying repeater is used.

It assumes that a transmitter includes two transmitting antennas, afirst antenna 1 and a second antenna 2. Also, the first antenna 1transmits a STBC applied signal u1 that is encoded based on STBC, andthe second antenna 2 transmits a STBC applied signal u₂.

In the transmission method shown in FIG. 3, the satellite 30 is alsoregarded as the first antenna and the repeaters 31 to 33 are regarded asthe second antenna. However, since the transmission bands of the link Cbetween the satellite 30 and the portable terminal 60 and the links B1,B2, and B3 are same, the satellite 30 transmits a signal u1 with thesame band through the links B1, B2, B3, and C. The repeaters 31 to 33receives the signal u₁ from the satellite 30, transform the receivedsignal u₁ to a signal u₂ having the same frequency band, and transmitthe transformed signal u₂ to the portable terminal through correspondinglinks D1, D2, and D3.

Herein, the portable terminal 60 receives the signal u₁ from thesatellite 30 and the signals u₂ from the repeaters 31 to 33 at the sametime, and detects a corresponding signal by performing the STBC decodingafter discriminating two STBC encoded signals by performing channelestimation and STBC decoding.

In the signal transmission scheme shown in FIG. 3, the repeaters 31 to33 transform the signal u₁ from the satellite 30 to the signal u₂ havingthe same frequency band, simply amplifying the transformed signal u₂,and relaying the amplified signal u₂ to the portable terminal 60. Thissignal transmission scheme may increase load due to signal processing ina satellite communication network. However, the repeaters 31 to 33 canbe effectively managed because the repeaters 31 to 33 use the samemethod to process the signals.

FIG. 4 is a diagram illustrating signals transmitted from a satelliteand repeaters in accordance with an embodiment of the present inventionfor describing that a transmitting method according to the presentembodiment can be applied to a satellite communication networksincluding a simple amplifying repeater and/or a frequency transformrepeater.

It assumes that a transmitter includes two transmitting antennas, afirst antenna 1 and a second antenna 2. Also, the first antenna 1transmits a STBC applied signal u₁ that is encoded based on STBC, andthe second antenna 2 transmits a STBC applied signal u₂.

Referring to FIG. 4, a transmission method for a satellite communicationnetwork including a simple amplifying repeater will be described.

A satellite transmits STBC encoded signals through a band of a linkformed between the satellite and a portable terminal and a band of alink formed between the satellite and repeaters. A predetermined simpleamplifying repeater, for example, a first simple amplifying repeater,among the repeaters receives the signal u₁ from the satellite throughthe link formed to the satellite, amplifies the received signal u₁, andtransmits the amplified signal u₁ to a portable terminal through a linkformed to the portable terminal. The other simple amplifying repeaters,for example, a second simple amplifying repeater, receives the u₁ signalfrom the satellite through a link formed to the satellite, transformsthe received signal u₁ to a signal u₂ having the same frequency band,amplifies the transformed signal u₂, and transmit the amplified signalu₂ to a portable terminal through a band of a link formed to theportable terminal.

Hereinafter, a transmission method for a satellite communication networkincluding a frequency transforming repeater when the signal transmissionbands of a link between a satellite and a portable terminal and a linkbetween a satellite and a repeater are different.

A satellite transmits a STBC encoded signal u₁ through the band of alink formed to a portable terminal and transmits a STBC encoded signalu₂ to each repeater through the band of a link formed to each repeater.A predetermined repeater among the repeaters, for example, a firstfrequency transform repeater, receives the signal u₂ through the band ofthe link formed to the satellite, transforms the received signal u₂ to asignal u₂ having a frequency band corresponding to a link between thesatellite and the portable terminal, and transmits the transformedsignal u₂ through the and of the link formed to the portable terminal.The other frequency transform repeaters, for example, a second frequencytransform repeater, receive the signal u₂ through the band of the linkformed to the satellite, transform the received signal u₂ to a signal u₁having a frequency band corresponding to the link formed between thesatellite and the portable terminal, and transmit the transformed signalu₁ through the band of the link formed to the portable terminal.

In the transmission methods described with reference to FIG. 4, thesatellite 40 and some repeaters, for example, a repeater 43 in FIG. 4,are regarded as the first antenna 1, and the other repeaters, forexample, repeaters 41 and 42, are regarded as the second antenna 2.

For example, the satellite 40 transmits the same signal u₁ through thebands of links B1, B2, B3, and C. Some repeaters, for example, arepeater 43, receive the signal u₁ from the satellite 40, and transmitthe received signal u₁ through the band of the link D3. Other repeatersreceive the signal u₁ from the satellite 40 and transmit the receivedsignal u₂ to the portable terminal through the bands of links D1 and D2.

Accordingly, the portable terminal 60 receives the signal u₁ from thesatellite 40 and the repeater 43 and the signal u₂ from the repeaters 41and 42 at the same time. Then, the portable terminal 60 discriminatestwo STBC encoded signals through channel estimation and STBC decodingand detects corresponding signals.

In the signal transmission method shown in FIG. 4, the repeater 43having a simple amplifying function transmits the signal u₁ although theband of the link C is not formed as like the urban environment oralthough the transmission signal is too weak. Therefore, the portableterminal 60 can obtain large diversity gain although the portableterminal 60 is located at the urban environment.

Hereinafter, a method of detecting a corresponding signal throughperforming STBC decoding on signals received from satellite and fromrepeaters at a portable terminal will be described with reference toFIG. 5.

FIG. 5 is a flowchart illustrating a method of detecting a signal byperforming STBC decoding on signals received from a satellite andrepeaters at a portable terminal employing a WCDMA based IMT-2000wireless interface specification in accordance with an embodiment of thepresent invention.

As shown in FIGS. 2 to 4, the satellite and the repeaters performdifferent transmission methods according to corresponding environments.However, a portable terminal simply processes the received signalwithout considering the transmission methods of the received signal.

That is, the portable terminal detects a signal by performing STBCdecoding on a signal r₁ received through the band of a link C formed tothe satellite and a signal r₂ received through the bands of links D1,D2, and D3 formed to the repeaters, and obtains diversity gain throughtemporal and spatial correlation.

A method of obtaining the diversity gain at the portable terminal isdefined in the known 3GPP terrestrial WCDMA based IMT-2000 wirelessinterface specification. Therefore, the detailed description thereof isomitted. As shown in FIGS. 2 to 4, the terminal receives the signal u1or the signal u₂ from the repeaters. Herein, a signal r₂ denotes asignal received from a repeater, and a signal r₁ denotes a signalreceived from a satellite.

In an embodiment of the present invention, an information series

x ₀ ={x ₀(m)}

which is a signal transmitted from a transmitting end such as asatellite and a repeater is encoded into two streams by a STBC encoder.Herein, if output signals are defined as

x₀ ¹=x₀, x₀ ²=j_(p)x₀

for STBC of

x_(O) ¹=[x_(O), x_(O)*]^(T)

having 2P samples, the two output signals can be expressed as followingEq. 1.

$\begin{matrix}{{X_{0} = {\begin{bmatrix}x_{0}^{1} \\x_{0}^{2}\end{bmatrix} = {\begin{bmatrix}x_{0} \\{J_{P}x_{0}^{*}}\end{bmatrix} = {{\begin{bmatrix}I_{P} & 0 \\0 & J_{P}\end{bmatrix}\begin{bmatrix}x_{0} \\x_{0}^{*}\end{bmatrix}} = {\Omega_{p}x_{0}^{\prime}}}}}}{where}{{\Omega_{p} = \begin{bmatrix}I_{P} & 0 \\0 & J_{P}\end{bmatrix}},{J_{P} = {I_{p} \otimes \begin{bmatrix}0 & 1 \\{- 1} & 0\end{bmatrix}}}}} & {{MathFigure}\mspace{14mu} 1}\end{matrix}$

In Eq. 1,

denotes a Kronecker Product.

In transmitting a signal by regarding a satellite and a repeater as twotransmitting antenna as shown in FIGS. 2 to 4, a FIR filter

hi_(,i)(n)

for L paths of a channel between a transmitting antenna i and areceiving antenna and a chip delay time T_(c) as a tap delay time can beexpressed as Eq. 2.

$\begin{matrix}{{h_{i,1}(n)} = {\sum\limits_{l = 0}^{L - 1}{{h_{i,1}^{l}(n)}{\delta \left( {n - l} \right)}}}} & {{MathFigure}\mspace{14mu} 2}\end{matrix}$

As described above, a transmitting end encodes the information sequenceas like Eq. 1 into two streams by performing the STBC encoding, and theSTBC encoded streams are spread by a channelization code and scrambledby a scrambling code in order to distinguish a transmitting spot and areceiving spot. A signal u₁ transmitted from a link regarded as thei^(th) transmitting antenna can be expressed as following Eq. 3.

$\begin{matrix}{u_{i} = \left\{ {{u_{i}(n)} = {{b(n)}{\sum\limits_{m = 0}^{P - 1}{{x_{0}^{i}(m)}{c\left( {n - {Gm}} \right)}}}}} \right\}} & {{MathFigure}\mspace{14mu} 3}\end{matrix}$

In Eq. 3, b(n) denotes a long complex scrambling code to identify atransmitting spot and a receiving spot, and c(n) denotes achannelization code having a length G.

In Eq. 3, if S^(G) is defined as a channelization code and a (G×1)vector having a complex scrambling code value, a channel sign matrix Scan be expressed as S=diag(S^(G)), and a signal u transmitted throughtwo streams using Eq. 1 can be expressed as following Eq. 4.

$\begin{matrix}{u = {\begin{bmatrix}u_{1} \\u_{2}\end{bmatrix} = {S\; \Omega_{P}x_{0}^{\prime}}}} & {{MathFigure}\mspace{14mu} 4}\end{matrix}$

Using a channel model between a transmitting antenna and a receivingantenna, a channel matrix

H₁ ^(i)(GP×GP)

can be expressed as a toeplitz matrix as like Eq. 5.

$\begin{matrix}{H_{1}^{i} = \begin{bmatrix}{h_{i,1}^{0}(0)} & 0 & \Lambda & \Lambda & 0 \\M & {h_{i,1}^{0}(1)} & \; & \; & M \\{h_{i,1}^{L - 1}(0)} & M & O & \; & M \\M & {h_{i,1}^{L - 1}(1)} & \; & O & M \\0 & M & \Lambda & \; & {h_{i,1}^{0}({GP})}\end{bmatrix}} & {{MathFigure}\mspace{14mu} 5}\end{matrix}$

A vector for a receiving signal received in one block unit at a terminalthrough Eq. 1 to Eq. 5 can be expressed as Eq. 6.

$\begin{matrix}{r = {\begin{bmatrix}r_{1} \\r_{2}\end{bmatrix} = {{\underset{= H}{\begin{bmatrix}H_{1}^{1} & H_{1}^{2}\end{bmatrix}}\underset{= u}{\begin{bmatrix}u_{1} \\u_{2}\end{bmatrix}}} + n}}} & {{MathFigure}\mspace{14mu} 6}\end{matrix}$

The terminal detects a transmitting signal

-   {tilde over (x)}′₀    by performing STCB decoding like Eq. 7 based on the receiving signal    of Eq. 6 and a channel matrix H through channel estimation like Eq.    5 as shown in steps S501, S502, S503, S504, and S505 in FIG. 5.

MathFigure 7

{tilde over (x)} ₀′=(HSΩ _(GP))^(H) r

In order to perform the STBC decoding to detect a signal at the stepS504, a terminal must know information about STBC encoding performed ina transmitting end such as a satellite or a repeater. Also, the terminalmust know information about a channel between the transmitting end and areceiving end such as the terminal for obtaining a channel matrix at thestep S503. It is preferable to exchange the channel information and theSTBC encoding information between the transmitting end and the receivingend at step S510.

FIG. 6 is a block diagram for describing channel estimation at aterminal in accordance with an embodiment of the present invention.

In order to deeply understand the channel estimation proposed in thepresent invention, the signal transmission method at the satellite andthe repeater shown in FIG. 4 is illustrated with the channel estimationthereof in FIG. 6.

As shown in FIG. 6, a satellite 40 transmits the same signal u₁ usingthe band of links B1, B2, B3, and C. A repeater 43 receives the signalu₁, simply amplifies the received signal u₁, and transmits the amplifiedsignal u₁ using the band of the link D3. The other repeaters 41 and 42receive the signal u₁ from the satellite 40, transform the receivedsignal u₁ to a signal u₂ in frequency, and transmit the signal u₂ usingthe bands of links D1 and D2.

The terminal performs channel estimation that compensates a channelthrough the channel estimation information. The channel estimation maybe performed at a terminal or a transmitting end. FIG. 6 shows thechannel estimation procedure performed in the terminal. Since theperformance of channel estimation greatly influences the STBC, channelestimation, compensation, and prediction algorithms of FIG. 6 areintroduced.

That is, the terminal estimates channel information of a path related tothe signals u₁ and u₂ using a pilot symbol which is previously receivedfrom a transmitting end as channel estimation information and well-knownalgorithms such as MMSE, Least square, and ML.

These channel estimation methods, however, are developed for aterrestrial mobile communication network. Since the signals have a longround trip delay time between a transmitter and a receiver, if thesechannel estimation methods are used in the terminal according to thepresent invention, the performance thereof becomes seriously degradeddue to the channel estimation error caused by the long delay time.

In an exemplary embodiment of the present invention, a channelestimation scheme that compensates the round-trip delay time forapplying STBC into a satellite communication network is proposed, andwill be described with reference FIG. 6.

In an exemplary embodiment of the present invention, a terminalestimates a channel estimation value of a next time point of receiving asignal using a channel estimation value before a long round-trip delaytime and a channel estimation value of a current time point of receivinga signal, where the channel estimation value before the long round-tripdelay time is the channel estimation value of a previous time point ofreceiving a STBC encoded signal. After estimating, the terminaltransmits the channel estimation value to a transmitting end such as asatellite or a repeater, preferably, a satellite.

That is, the terminal estimates the channel information value for areceiving signal as follows. The terminal calculates a firstcompensation value by obtaining a channel estimation value using aconventional channel estimation method, delaying the obtained channelestimation value as much as D′ applying a coefficient α to the delayedchannel estimation value, and subtracting the result thereof from theobtained channel estimation value. Then, the terminal calculates asecond compensation value by delaying the first compensation value asmuch as D and applying a coefficient β into the delayed compensationvalue. Afterward, the first compensation value and the secondcompensation value are added to estimate the channel information valuefor the receiving signal. Such a channel estimation scheme is performedon each of received signals u₁ and u₂.

Especially, if α, β, D and D′ are set up as 0, it is equivalent to aconventional channel compensation scheme. In an exemplary embodiment ofthe present invention, D and D′ are setup with very small values, and αand β are setup with proper values. Then, the terminal can accuratelyestimate channel information through channel information correlation toprevious time.

In an exemplary embodiment of the present invention, a channelestimation scheme may be performed in a transmitting end. That is, thetransmitting end previously compensates a channel before transmitting asignal to a terminal as follows.

A terminal estimates channel information of a received signal uses apilot symbol or a conventional channel estimation scheme, and feeds theestimated channel information back to the transmitting end. Thetransmitting end calculates a first compensation value by delaying thereceived channel estimation value as long as D′ applying a coefficient αto the delayed channel estimation value, and subtracting the result fromthe received channel estimation value. Then, the transmitting endcalculates a second compensation value by delaying the firstcompensation value as long as D and applying a coefficient into thedelayed compensation value. Afterward, the first compensation value andthe second compensation value are added to estimate the channelinformation value for the transmitting signal. Such a channel estimationscheme is performed on each of transmitting signals u₁ and u₂. Thechannel estimation scheme for the transmitting end is identical to thatfor the terminal. By performing the channel estimation scheme accordingto an exemplary embodiment of the present invention at the transmittingend, the load of the terminal for the channel estimation can be reduced.

The channel estimation scheme according to an exemplary embodiment ofthe present invention can be effectively used even in an environment inwhich a transmitting end cannot compensate a channel because a channelof a receiving end quickly changes, for example, when the terminal istraveling in very high speed in a terrestrial mobile communicationnetwork.

Although the present invention is described using STBC as an example inthe exemplary embodiments of the present invention shown in FIGS. 1 to6, it is obvious to those skilled in the art that the present inventioncan be applied to any satellite communication system includingrepeaters, and multiple input and multiple output (MIMO) system, and anytemporal and spatial code used to obtain temporal and spatial diversitygain can be used.

The above described method according to the present invention can beembodied as a program and stored on a computer readable recordingmedium. The computer readable recording medium is any data storagedevice that can store data which can be thereafter read by the computersystem. The computer readable recording medium includes a read-onlymemory (ROM), a random-access memory (RAM), a CD-ROM, a floppy disk, ahard disk and an optical magnetic disk.

The present application contains subject matter related to Korean patentapplication No. 2006-0091983, filed in the Korean Intellectual PropertyOffice on Sep. 21, 2006, the entire contents of which is incorporatedherein by reference.

While the present invention has been described with respect to certainpreferred embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the scope of the invention as defined in the following claims.

1. A satellite communication system for obtaining a diversity gain bytransmitting a signal applied with a space and time block code (STBC)when a signal transmission band of a link between a satellite and aterminal differs from that of a link between a satellite and a repeater,comprising: a satellite for transmitting a first signal encoded to STBCusing a band of a link formed between the satellite to a terminal andtransmitting a second signal encoded to STBC using a band of a linkformed between the satellite to a repeater; and at least one offrequency transforming repeaters for receiving the second signal throughthe band of the link formed between the satellite to the repeater,transforming the received second signal to a signal with other frequencyband, and transmitting the transformed second signal using a band of alink formed to a terminal.
 2. The satellite communication system asrecited in claim 1, wherein the terminal distinguishes a signal receivedfrom a satellite from a signal received from a repeater, and detects acorresponding signal through performing STBC decoding.
 3. The satellitecommunication system as recited in claim 1, wherein the terminalestimates a channel estimation value of a next time point of receiving asignal using a channel estimation value of a previous time point ofreceiving a signal from a satellite and a repeater and another channelestimation value of a current time point of receiving a signal, andfeeds the estimated channel estimation value back to the satellite orthe repeater.
 4. A satellite communication system for obtaining adiversity gain by transmitting a signal applied with a space and timeblock code (STBC) when a signal transmission band of a link between asatellite and a terminal is identical to that of a link between asatellite and a repeater, comprising: a satellite for transmitting afirst signal encoded to STBC using a band of a link formed between thesatellite to a terminal and using a band of a link formed between thesatellite to each repeater, respectively; and at least one of simpleamplifying repeaters for receiving the first signal through the band ofthe link formed between the satellite and the repeater, transforming thereceived first signal to a second signal having the same frequency bandof the first signal, and transmitting the transformed second signalusing a band of a link formed to a terminal.
 5. The satellitecommunication system as recited in claim 4, wherein the terminaldistinguishes a signal received from a satellite from a signal receivedfrom a repeater, and detects a corresponding signal through performingSTBC decoding.
 6. The satellite communication system as recited in claim4, wherein the terminal estimates a channel estimation value of a nexttime point of receiving a signal using a channel estimation value of aprevious time point of receiving a signal from a satellite and arepeater and another channel estimation value of a current time point ofreceiving a signal, and feeds the estimated channel estimation valueback to the satellite or the repeater.
 7. A satellite communicationsystem for obtaining a diversity gain by transmitting a signal appliedwith a space and time block code (STBC) when a satellite communicationnetwork include repeaters having a simple amplifying function,comprising: a satellite for transmitting a first signal encoded to STBCusing a band of a link formed between the satellite to a terminal andusing a band of a link formed between the satellite to each repeater,respectively; at least one of first simple amplifying repeaters forreceiving the first signal through the band of the link formed to thesatellite, amplifying the received first signal, and transmitting theamplified first signal using a band of a link formed to a terminal; andat least one of second simple amplifying repeater for receiving thefirst signal through the band of the link formed to the satellite,transforming the first signal to a second signal having the samefrequency band of the first signal, amplifying the transformed secondsignal, and transmitting the amplified second signal using a band of alink formed to a terminal.
 8. The satellite communication system asrecited in claim 7, wherein the terminal distinguishes a signal receivedfrom a satellite from a signal received from a repeater, and detects acorresponding signal through performing STBC decoding.
 9. The satellitecommunication system as recited in claim 7, wherein the terminalestimates a channel estimation value of a next time point of receiving asignal using a channel estimation value of a previous time point ofreceiving a signal from a satellite and a repeater and another channelestimation value of a current time point of receiving a signal, andfeeds the estimated channel estimation value back to the satellite orthe repeater.
 10. A satellite communication system for obtaining adiversity gain by transmitting a signal applied with a space and timeblock code (STBC) when a satellite communication network includerepeaters having a frequency transforming function and a signaltransmission band of a link between a satellite and a terminal differsfrom that of a link between a satellite and a repeater, comprising: asatellite for transmitting a first signal encoded to STBC using a bandof a link between the satellite to a terminal and transmitting a secondsignal that is encoded to STBC using a band of a link formed between thesatellite to a repeater; at least one of first frequency transformingrepeaters for receiving the second signal through the band of the linkto the satellite, transforming the received second signal to a secondsignal having a frequency band corresponding to the link between thesatellite and a terminal, and transmitting the transformed second signalusing a band of a link formed to a terminal; and at least one of secondfrequency transforming repeater for receiving the second signal throughthe band of the link formed to the satellite, transforming the receivedsecond signal to a first signal having a frequency band corresponding toa link between the satellite and the terminal, and transmitting thetransformed first signal using a band of a link formed to a terminal.11. The satellite communication system as recited in claim 10, whereinthe terminal distinguishes a signal received from a satellite from asignal received from a repeater, and detects a corresponding signalthrough performing STBC decoding.
 12. The satellite communication systemas recited in claim 10, wherein the terminal estimates a channelestimation value of a next time point of receiving a signal using achannel estimation value of a previous time point of receiving a signalfrom a satellite and a repeater and another channel estimation value ofa current time point of receiving a signal, and feeds the estimatedchannel estimation value back to the satellite or the repeater.
 13. Amethod for detecting a signal in a terminal in a satellite communicationsystem for obtaining a diversity gain by transmitting a signal appliedwith space time block code (STBC) from a satellite and a repeater to theterminal, the method comprising the steps of: estimating channelinformation corresponding to each received signal when receiving asignal encoded to STBC from a satellite and a repeater at the same time;generating a channel matrix using the estimated channel information; anddetecting a transmitting signal by performing STBC decoding on thereceived signal based on STBC encoding information previously receivedfrom a satellite and a repeater and the generated channel matrix.
 14. Amethod for estimating a channel in a terminal in a satellitecommunication system for obtaining a diversity gain by transmitting asignal applied with space time block code (STBC) from a satellite and arepeater to the terminal, the method comprising the steps of: estimatingchannel information value by performing a predetermined channelestimation scheme on signals received from a satellite and a repeater;delaying the estimated channel information value as much as D′ applyingto the delayed channel information value, and subtracting the result ofapplying the coefficient α from the estimated channel information value;and delaying the subtracted channel information value as much as D,applying to the delayed channel information value, and adding the resultof applying the coefficient β to the subtracted channel informationvalue.